Influence of ion size on the stability of chloroplumbates containing PbCl<Stack><Subscript>5</Subscript><Superscript>−</Superscript></Stack> or PbCl<Stack><Subscript>6</Subscript><Superscript>2−</Superscript></Stack>

The enthalpies of formation of PbCl₄, PbCl₅⁻ and PbCl₆²⁻, originating from quantum mechanics, have enabled the thermodynamic behaviour of these ions with respect to Cl-detachment to be assessed. The stability of salts containing PbCl₅⁻ and PbCl₆²⁻ as a function of the dimensions of these anions and complementary cations was studied using an approach combining the Kapustinskii-Yatsimirskii equation with basic thermochemical relationships. It was found that hexachloroplumbates of monovalent metal cations will not dissociate into metal chlorides and PbCl₄, provided the complementary cations are suitably large in size. Hexachloroplumbates of divalent metal cations have not yet been synthesised since no known metal cations attain the requisite large size. Such salts will not dissociate if the divalent metal cations are able to complex suitably large electron-donating ligands. The pentachloroplumbates of both monovalent and divalent metal cations are unstable, since no known metal cations have appropriately large ionic radii. The approach adopted appears to be useful for the examination of the thermal behaviour, stability and reactivity of chloroplumbates.     

Global and local interactions in the structure of crystalline 7-(diethylamino)-2-(2-oxo-2<Emphasis Type="Italic">H</Emphasis>-chromen-3-yl)chromenium perchlorate

Computational methods were used to calculate the crystal lattice energy reflecting global interactions, predominantly long-range electrostatic interactions between ions, as well as the energy of selected specific local C–H···O, C–H···π and π···π interactions found in synthesized 7-(diethylamino)-2-(2-oxo-2H-chromen-3-yl)chromenium perchlorate, the structure of which was determined by X-ray crystallography. Local interactions occurring between specific sites of molecules, amounting to a few tens of kJ mol^?1, most likely account for the mutual arrangement of molecular ions, whereas global ones, exceeding half-a-thousand kJ mol^?1, are responsible for the thermodynamic stability of the compound investigated in the crystalline solid phase, whose potential applications are briefly outlined.     

Thermodynamics and Kinetics of the Thermal Decomposition of N,N,N-trimethylmethanaminium and 1-methylpyridinium Halides

The decomposition of the quaternary salts mentioned in the title was examined at the quantum mechanical Hartree-Fock level of theory employing pseudopotentials combined with a SBKJ** basis set. This enabled identification of intermediate and transition state species on the reaction pathway and prediction of the thermodynamic and kinetic barriers to the dissociation of the compounds in the gaseous phase. Application of classical methods permitted the lattice energies of salts, whose crystal structures had been established earlier, to be predicted. Combination of these latter characteristics with the heats of formation of gaseous halide ions (available from the literature) and the relevant cations (obtained at the density functional (B 3LYP)/6-31G**level of theory) provided heats of formation of the salts. On the basis of these values, the thermodynamic and kinetic barriers to the dissociation of the compounds were predicted. The characteristics thus obtained compare quite well with those available in the literature or determined in this work on the basis of TG or DSC measurements. These investigations have shed more light on the mechanism of the thermal dissociation of quaternary salts, and more generally on thermal processes involving solids. This revised version was published online in August 2006 with corrections to the Cover Date.     

Lattice energetics and thermochemistry of phenyl acridine-9-carboxylates and 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonates

The melting points and melting enthalpies of nine phenyl acridine-9-carboxylates—nitro-, methoxy- or halogen-substituted in the phenyl fragment—and their 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonate derivatives were determined by DSC. The volatilisation temperatures and enthalpies of phenyl acridine-9-carboxylates were either measured by DSC or obtained by fitting TG curves to the Clausius–Clapeyron relationship. For the compounds whose crystal structures are known, crystal lattice energies and enthalpies were determined computationally as the sum of electrostatic, dispersive and repulsive interactions. By combining the enthalpies of formation of gaseous phenyl acridine-9-carboxylates or 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonate ions, obtained by the DFT method, and the corresponding enthalpies of sublimation and/or crystal lattice enthalpies, the enthalpies of formation of the compounds in the solid phase were predicted. In the case of the phenyl acridine-9-carboxylates, the computationally predicted crystal lattice enthalpies correspond reasonably well with the experimentally obtained enthalpies of sublimation. The crystal lattices of phenyl acridine-9-carboxylates are stabilised predominantly by dispersive interactions between molecules, whilst the crystal lattices of their quaternary salts are stabilised by electrostatic interactions between ions.     

Melting,volatilisation and crystal lattice enthalpies of acridin-9(10H)-ones

The enthalpies and temperatures of melting and sublimation of acridin-9(10H)-one, 10-methylacridin-9(10H)-one, 2,10-dimethylacridin-9(10H)-one, 10-methyl-2-nitroacridin-9(10H)-one, 10-ethylacridin-9(10H)-one and 10-phenylacridin-9(10H)-one were measured by DSC. Enthalpies and temperatures of volatilisation were also obtained by fitting TG curves to the Clausius-Clapeyron relationship. Complementary investigations for anthracene showed the extent to which the thermodynamic characteristics thus obtained compare with those determined by means of other techniques. For compounds whose crystal structures are known, experimental enthalpies of sublimation correspond reasonably well to crystal lattice enthalpies predicted theoretically as the sum of electrostatic, dispersive and repulsive interactions. Analysis of crystal lattice enthalpy contributions indicates that dispersive interactions always predominate. Interactions are enhanced in acridin-9(10H)-one where intermolecular hydrogen bonds occur: this is reflected in the relatively high enthalpy of sublimation. This revised version was published online in July 2006 with corrections to the Cover Date.     

TG-FTIR, DSC, and quantum-chemical studies on the thermal decomposition of quaternary ethylammonium halides

The thermal decomposition of quaternary ethylammonium chloride, bromide, and iodide has been studied using the experimental techniques of thermal gravimetry coupled to Fourier transform infrared spectroscopy (TG-FTIR) and differential scanning calorimetry (DSC) as well as the density functional theory (DFT) and MP2 quantum-chemical methods. These compounds decompose in a one-step process, and the almost perfect agreement between the experimental IR spectra and those predicted at the B3LYP/6-311G(d,p) level demonstrates for the first time that decomposition produces an equimolar mixture of triethylamine and a haloethane. The respective experimental enthalpies of dissociation of the chloride, bromide, and iodide are 158, 181, and 195 kJ/mol. These values correlate well with the calculated enthalpies of dissociation based on crystal lattice energies and quantum-chemical thermodynamic barriers. The experimental activation barriers were derived from the least-squares fit of the F1 kinetic model (first-order process) to thermogravimetric traces. These estimates are 184, 286, and 387 kJ/mol for chloride, bromide, and iodide, respectively, and agree well with the theoretical calculations. It has been demonstrated that the theoretical approach assumed in this work is capable of predicting the relevant characteristics of the thermal decomposition of solids with experimental accuracy. DFT methodology is recommended for the quantum-chemical part of the model: B3LYP for evaluating the thermodynamic barriers and MPW1K for assessing the activation characteristics. These quantum-chemical data then have to be combined with crystal lattice energies. The latter should be calculated using both electrostatic and repulsion-dispersion terms.     

AT base pair anions versus (9-methyl-A)(1-methyl-T) base pair anions

The anionic base pairs of adenine and thymine, (AT)(-), and 9-methyladenine and 1-methylthymine, (MAMT)(-), have been investigated both theoretically and experimentally in a complementary, synergistic study. Calculations on (AT)(-) found that it had undergone a barrier-free proton transfer (BFPT) similar to that seen in other dimer anion systems and that its structural configuration was neither Watson-Crick (WC) nor Hoogsteen (HS). The vertical detachment energy (VDE) of (AT)(-) was determined by anion photoelectron spectroscopy and found to be in agreement with the VDE value predicted by theory for the BFPT mechanism. An AT pair in DNA is structurally immobilized into the WC configuration, in part, by being bonded to the sugars of the double helix. This circumstance was mimicked by methylating the sites on both A and T where these sugars would have been tied, viz., 9-methyladenine and 1-methylthymine. Calculations found no BFPT in (MAMT)(-) and a resulting (MAMT)(-) configuration that was either HS or WC, with the configurations differing in stability by ca. 2 kcal/mol. The photoelectron spectrum of (MAMT)(-) occurred at a completely different electron binding energy than had (AT)(-). Moreover, the VDE value of (MAMT)(-) was in agreement with that predicted by theory. The configuration of (MAMT)(-) and its lack of electron-induced proton transfer are inter-related. While there may be other pathways for electron-induced DNA alterations, BFPT in the WC/HS configurations of (AT)(-) is not feasible.     

Theoretical and Experimental Investigations of The Decomposition of 10-methylacridinium Halides

10-Methylacridinium chloride, bromide and iodide were prepared in crystalline forms (the first two salts as monohydrates) and subjected to thermogravimetric investigations. Decomposition of the compounds is initially accompanied by the liberation of water (in case of monohydrates), halomethanes and acridine molecules. As decomposition proceeds, side reactions occur which are reflected in a complex pattern of thermogravimetric curves. TG traces corresponding to the initial decomposition stage were used to determine the kinetic characteristics of the thermal dissociation of the salts. MNDO/d, AM1 and PM3 methods were employed independently to examine reaction pathways and to predict thermodynamic and kinetic barriers for the thermal decomposition of the compounds. These data were subsequently supplemented with theoretically determined crystal lattice energies, which enabled the relevant characteristics for the decomposition of crystalline phases to be predicted. The theoretically predicted characteristics are qualitatively comparable with those originating from thermogravimetric investigations, which allows one to believe that both are valid. This revised version was published online in August 2006 with corrections to the Cover Date.     

Thermochemistry and crystal lattice energetics of phenyl acridine-9-carboxylates and 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonates

The melting enthalpies and melting points of phenyl acridine-9-carboxylate, its eleven alkyl-substituted derivatives in the phenyl fragment and eight 9-phenoxycarbonyl-10-methylacridinium trifluoromethanesulphonates derived from them, were determined by DSC. The volatilisation enthalpies and temperatures of twelve phenyl acridine-9-carboxylates were either measured by DSC or obtained by fitting TG curves to the Clausius–Clapeyron relationship. For the compounds whose crystal structures are known, crystal lattice enthalpies were determined computationally as the sum of electrostatic, dispersive and repulsive interactions. By combining the enthalpies of formation of gaseous phenyl acridine-9-carboxylates or 9-phenoxycarbonyl-10-methylacridinium and trifluoromethanesulphonate ions, obtained by quantum chemistry methods, and the corresponding enthalpies of sublimation or crystal lattice enthalpies, the enthalpies of formation of the compounds in the solid phase were predicted. In the case of the phenyl acridine-9-carboxylates, the computationally predicted crystal lattice enthalpies correspond reasonably well to the experimentally obtained enthalpies of sublimation. Analysis of crystal lattice enthalpy contributions indicates that the crystal lattices of phenyl acridine-9-carboxylates are stabilised predominantly by dispersive interactions between molecules, whereas the crystal lattices of their quaternary salts are stabilised by electrostatic interactions between ions.     

Enthalpies of Sublimation and Crystal Lattice Energies of 9-Acridinamine and its Derivatives

Enthalpies of sublimation of acridine, 9-acridinamine, N-methyl-9-acridinamine, 10-methyl-9-acridinimine, N,N-dimethyl-9-acridinamine and N-methyl-10-methyl-9-acridinimine were determined by fitting to thermogravimetric curves with the Clausius-Clapeyron relationship. These values compare well with crystal lattice energies predicted theoretically as the sum of electrostatic, dispersive and repulsive interactions. Partial charges for these calculations were obtained on an ab initio level, while atomic parameters were taken from literature. This revised version was published online in August 2006 with corrections to the Cover Date.